Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants.
OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.
One application for phosphorescent emissive molecules is a full color display. Industry standards for such a display call for pixels adapted to emit particular colors, referred to as “saturated” colors. In particular, these standards call for saturated red, green, and blue pixels. Color may be measured using CIE coordinates, which are well known to the art.
One example of a green emissive molecule is tris(2-phenylpyridine) iridium, denoted Ir(ppy)3, which has the following structure:

In this, and later figures herein, we depict the dative bond from nitrogen to metal (here, Ir) as a straight line.
As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule,” and it is believed that all dendrimers currently used in the field of OLEDs are small molecules.
As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between.
As used herein, “solution processible” means capable of being dissolved, dispersed, or transported in and/or deposited from a liquid medium, either in solution or suspension form.
A ligand may be referred to as “photoactive” when it is believed that the ligand directly contributes to the photoactive properties of an emissive material. A ligand may be referred to as “ancillary” when it is believed that the ligand does not contribute to the photoactive properties of an emissive material, although an ancillary ligand may alter the properties of a photoactive ligand.
As used herein, and as would be generally understood by one skilled in the art, a first “Highest Occupied Molecular Orbital” (HOMO) or “Lowest Unoccupied Molecular Orbital” (LUMO) energy level is “greater than” or “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level. Since ionization potentials (IP) are measured as a negative energy relative to a vacuum level, a higher HOMO energy level corresponds to an IP having a smaller absolute value (an IP that is less negative). Similarly, a higher LUMO energy level corresponds to an electron affinity (EA) having a smaller absolute value (an EA that is less negative). On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material. A “higher” HOMO or LUMO energy level appears closer to the top of such a diagram than a “lower” HOMO or LUMO energy level.
As used herein, and as would be generally understood by one skilled in the art, a first work function is “greater than” or “higher than” a second work function if the first work function has a higher absolute value. Because work functions are generally measured as negative numbers relative to vacuum level, this means that a “higher” work function is more negative. On a conventional energy level diagram, with the vacuum level at the top, a “higher” work function is illustrated as further away from the vacuum level in the downward direction. Thus, the definitions of HOMO and LUMO energy levels follow a different convention than work functions.
More details on OLEDs, and the definitions described above, can be found in U.S. Pat. No. 7,279,704, which is incorporated herein by reference in its entirety.
Throughout the present disclosure, reference will be made to the following references, which are hereby incorporated by reference in their entireties:
[1] M. A. Baldo, D. F. O'brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, and S. R. Forrest, Highly efficient phosphorescent emission from organic electroluminescent devices,” Nature, vol. 395, no. 6698, pp. 151-154, 1998.
[2] K. Saxena, V. K. Jain, and D. S. Mehta, “A review on the light extraction techniques in organic electroluminescent devices,” Opt. Materials, vol. 32, no. 1, pp. 221-233, 2009.
[3] W. Brutting, J. Frischeisen, T. D. Schmidt, B. J. Scholz, and C. Mayr, “Device efficiency of organic light-emitting diodes: Progress by improved light outcoupling,” Phys. Status Solidi A, vol. 210, no. 1, pp. 44-65, 2013.
[4] V. Bulovic, V. B. Khalfin, G. Gu, P. E. Burrows, D. Z. Garbuzov, and S. R. Forrest, “Weak microcavity effects in organic light-emitting devices,” Phys. Rev. B, vol. 58, no. 7, pp. 3730-40, 1998.
[5] S. Nowy, B. C. Krummacher, J. Frischeisen, N. A. Reinke, and W. Brutting, “Light extraction and optical loss mechanisms in organic light-emitting diodes: Influence of the emitter quantum efficiency,” J. Appl. Phys., vol. 104, no. 123109, 2008.
[6] K. Y. Yang, K. C. Choi, and C. W. Ahn, “Surface plasmon-enhanced spontaneous emission rate in an organic lightemitting device structure: Cathode structure for plasmonic application,” Appl. Phys. Lett., vol. 94, no. 173301, 2009.
[7] M. Furno, R. Meerheim, S. Hofmann, B. Lussem, and K. Leo, “Efficiency and rate of spontaneous emission in organic electroluminescent devices,” Phys. Rev. B, vol. 85, no. 115205, 2012.
[8] A. F. Oskooi, D. Roundy, M. Ibanescu, P. Bermel, J. D. Joannopoulos, and S. G. Johnson, “MEEP: A flexible free-software package for electromagnetic simulations by the FDTD method,” Computer Physics Communications, vol. 181, pp. 687-702, 2010.
[9] A. Taflove, A. Oskooi, and S. G. Johnson, eds., Advances in FDTD Computational Electrodynamics: Photonics and Nanotechnology. Boston: Artech House, 2013.
[10] W. L. Barnes, “Fluorescence near interfaces: the role of photonic mode density,” J. Mod. Opt., vol. 45, no. 4, pp. 661-699, 1998.
[11] J. M. Lupton, B. J. Matterson, I. D. W. Samuel, M. J. Jory, and W. L. Barnes, “Bragg scattering from periodically microstructured light emitting diodes,” Appl. Phys. Lett., vol. 77, no. 21, pp. 3340-3342, 2000.
[12] P. A. Hobson, S. Wedge, J. A. E. Wasey, I. Sage, and W. L. Barnes, “Surface plasmon mediated emission from organic light-emitting diodes,” Adv. Materials, vol. 14, no. 19, pp. 1393-1396, 2002.
[13] U. Geyer, J. Hauss, B. Riedel, S. Gleiss, U. Lemmer, and M. Gerken, “Large-scale patterning of indium tin oxide electrodes for guided mode extraction from organic light-emitting diodes,” J. Appl. Phys., vol. 104, no. 093111, 1998.
[14] J. Frischeisen, Q. Niu, A. Abdellah, J. B. Kinzel, R. Gehlhaar, G. Scarpa, C. Adachi, P. Lugli, and W. Brutting, “Light extraction from surface plasmons and waveguide modes in an organic light-emitting layer by nanoimprinted gratings,” Opt. Express, vol. 19, pp. A7-A19, 2010.
[15] C. S. Choi, D.-Y. Kim, S.-M. Lee, M. S. Lim, K. C. Choi, H. Cho, T.-W. Koh, and S. Yoo, “Blur-free outcoupling enhancement in transparent organic light emitting diodes: a nanostructure extracting surface plasmon modes,” Adv. Opt. Materials, vol. 1, no. 10, pp. 687-691, 2013.
[16] Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson, and S. R. Forrest, “Management of singlet and triplet excitons for efficient white organic light-emitting devices,” Nature, vol. 440, no. 7086, pp. 908-912, 2006.
[17] A. Oskooi, P. Favuzzi, Y. Tanaka, H. Shigeta, Y. Kawakami, and S. Noda, “Partially disordered photonic-crystal thin films for enhanced and robust photovoltaics,” Appl. Phys. Lett., vol. 100, no. 181110, 2012.
[18] A. Oskooi, Y. Tanaka, and S. Noda, “Tandem photonic-crystal thin-films surpassing Lambertian light-trapping limit over broad bandwidth and angular range,” Appl. Phys. Lett., vol. 104, no. 010121, 2014.
[19] A. Oskooi, M. De Zoysa, K. Ishizaki, and S. Noda, “Experimental demonstration of quasiresonant absorption in silicon thin films for enhanced solar light trapping,” ACS Photonics, vol. 1, pp. 304-309, 2014.
[20] A. D. Rakic, A. B. Djurisic, J. M. Elazar, and M. L. Majewski, “Optical properties of metallic films for vertical-cavity optoelectronic devices,” Appl. Opt., vol. 37, no. 22, pp. 5271-5283, 1998.
[21] A. Oskooi and S. G. Johnson, Advances in Computational Electrodynamics: Photonics and Nanotechnology, ch. 4: Electromagnetic Wave Source Conditions. Boston: Artech, 2013.
[22] P. W. Milonni, “Semiclassical and quantum-electrodynamical approaches in nonrelativistic radiation theory,” Physics Reports, vol. 25, pp. 1-81, 1976.
[23] F. Wijnands, J. B. Pendry, F. J. Garcia-Vidal, P. M. Bell, P. J. Roberts, and L. M. Moreno, “Green's functions for Maxwell's equations: application to spontaneous emission,” Optical and Quantum Electron., vol. 29, pp. 199-216, 1997.
[24] Y. Xu, R. K. Lee, and A. Yariv, “Quantum analysis and the classical analysis of spontaneous emission in a microcavity,” Phys. Rev. A, vol. 61, no. 033807, 2000.
[25] H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings. New York: Springer-Verlag, 1988.
[26] R. M. Amos and W. L. Barnes, “Modification of spontaneous emission lifetimes in the presence of corrugated metallic surfaces,” Phys. Rev. B, vol. 59, no. 7708, 1999.
[27] E. Fort and S. Gressilon, “Surface enhanced fluorescence,” J. Phys. D: Appl. Phys., vol. 41, no. 013001, 2008.
[28] J. Burke, G. Stegeman, and T. Tamir, “Surface-polariton-like waves guided by thin, lossy metal films,” Phys. Rev. B, vol. 33, no. 8, pp. 5186-5201, 1986.
[29] K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and A. Scherer, “Surface-plasmonenhanced light emitters based on InGaN quantum wells,” Nat. Materials, vol. 3, pp. 601-605, 2005.
[30] W. H. Koo, S. M. Jeong, F. Araoka, K. Ishikawa, S. Nishimura, T. Toyooka, and H. Takezoe, “Light extraction from organic light-emitting diodes enhanced by spontaneously formed buckles,” Nat. Photonics, vol. 4, pp. 222-226, 2010.
The development of organic light-emitting diodes (OLEDs), particularly those based on small-molecule phosphorescent materials [1], has led to a number of commercial applications arising from their nearly 100% internal quantum efficiency (IQE). Yet the external quantum efficiency (EQE) which takes into account the light-extraction efficiency still has significant room for improvement despite intensive work based on a number of different designs including microlens arrays, low-index microstructured grids, high-index substrates, orienteddipole emitters, photonic crystals, and plasmonic out-coupling schemes [2, 3].
What is needed in the art is a design strategy for enhancing both the light-extraction efficiency and the spontaneous-emission rate of the excitons [4-7] in nanostructured, white-emitting OLEDs (WOLEDs) operating under the conditions of broad spectral bandwidth and isotropic emitters.